Work this year developed more accurate methods to study platelet concentration in blood as it flows through capacity tubes. Previous work showed flows of whole anticoagulated blood have increased platelet concentrations in samples of tube blood obtained after instantaneously stopping flow. (For more information, see Biorheology, Vol. 17, 1980). Major changes include optimization of electronic platelet counting in small samples of whole blood and the use of prostaglandin E1 and chlorpromazine in the anticoagulant mixture. With these changes, the coefficient of variation is 0.05 for a typical, single condition of blood flow (normal platelet count equals 3 to 5 x 10 to the fifth power/mm, hematocrit equals 35, inlet wall shear rate equals 800 sec to the negative first power, inner diameter of tube equals 196 um and tube length equals 10cm); previous values were greater than 0.2. Examination of models that explain the elevated platelet concentration suggests use of phenomenological law of platelet flux that has a term proportional to the hematocrit gradient (in addition to a standard term proportional to the platelet gradient). Work on direct visualization of the flow via fluorescent techniques was begun. Via such methods, platelet locations in the tube flow experiments can be observed, and the results used to determine appropriate coefficients and gradients for models of platelet transport. The goal of this work is a more complete understanding of flow effects on platelet concentration near surfaces and, therefore, of flow effects in blood-surface interaction.